mfd: mc13xxx-core: Fix PMIC shutdown when reading ADC values

When trying to read any MC13892 ADC channel on a imx51-babbage board:

The MC13892 PMIC shutdowns completely.

After debugging this issue and comparing the MC13892 and MC13783
initializations done in the vendor kernel, it was noticed that the
CHRGRAWDIV bit of the ADC0 register was not being set.

This bit is set by default after power on, but the driver was
clearing it.

After setting this bit it is possible to read the ADC values correctly.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>